In 1963 a devastating earthquake struck Skopje, Yugoslavia, destroying
a large part of the city. Not only was the city's economic, social,
and political life virtually paralyzedits cultural property loss
was catastrophic.

Skopje, today the capital of the Former Yugoslav Republic of Macedonia,
is in the midst of one of the most seismically active areas of the
world, a region that stretches from the western end of the Mediterranean
Sea through North Africa, Italy, the Balkans, Greece, and Turkey,
and on into central Asia. Temples, monuments, defensive structures,
and buildings have all been subjected to destructive earthquakes
that have raked the area for countless centuries.

The southern Balkan region historically has been ravaged not only
by quakes but by a series of invading armies that preceded the establishment
of a variety of cultures whose art and architecture survive to this
day. Important remnants of this heritage include numerous Byzantine
churches dating from the 9th to the 14th century. The interiors
of these churches, still in daily use, were originally covered with
frescoes, many of which have survived. The earthquake risk remains
for these structures, as well as for other religious and historical
monuments. Their destruction or damage would constitute an irreplaceable
loss not only to the culture of the region but to the world.

The Skopje Project

The 14th Century Byzantine Church of St. Nikita,
located near Skopje. Photo: William S. Ginell

Since 1985 the Getty Conservation Institute has promoted emergency
preparedness and the development of preventive measures to protect
cultural heritage from disasters such as earthquakes. As part of
its efforts, the GCI in 1990 established a collaborative research
program with the Institute of Earthquake Engineering and Engineering
Seismology (IZIIS) of the University "St. Cyril and Methodius" and
the Republic Institute for the Protection of Cultural Monuments
(RZZSK) in Skopje to study seismic stabilization of Byzantine churches.
These institutes offered the expertise of highly experienced professionals
and modern experimental facilities to test retrofitting measures
that could benefit structures in the Balkans and elsewhere.

Dr. Predrag Gavrilovic, Professor of Seismic Engineering at IZIIS,
and Lazar Sumanov, a conservation architect and Deputy Director
of RZZSK, were the project's principal investigators. Together they
assembled a team of specialists in art history, architectural conservation,
chemistry, archaeology, geophysics, and engineering. From the beginning,
the objective was to develop methods that minimized physical intervention
and that preserved the cultural values of the churches.

Typical Byzantine construction in the Balkan region is characterized
by walls consisting of two outer faces of stone and brick set in
lime mortar as well as a core between the faces filled with rubble
set in lime mortar. About every meter up the wall were horizontal
belts of brick or timber which provided continuity and ductility
to the structure. Because the rubble cores of the walls are structurally
weak, building stability was achieved by making walls and mortar
joints very thick. However, as the mortar deteriorated with age
and the timbers rotted away, the buildings' vulnerability to earthquakes
increased.

The Skopje research program had three phases. In the first phase,
data were collected on 50 existing Byzantine churches, including
information on the topology of the structures, past conservation
efforts, soil and meteorological conditions, and the historic, cultural,
and artistic features of the churches. Preliminary structural analyses
were made and possible repair and strengthening concepts explored.
Ultimately, four churches were selected for detailed investigation,
each representing a major type of Byzantine church architecture
typical of the region: basilica, single nave, multidome apses, and
single or five-dome churches with a cross-shaped interior (an "inscribed
cross"). Most of the 50 churches studied fell into the last category.

The program's second phase included: definition of seismic parameters
for the four specific church locations, studies of the physical-chemical
properties and bearing capacity of building materials, experimental
measurements of the dynamic characteristics of selected structures,
and development of approaches for repair and strengthening. A major
component of this phase was testing, on a seismic simulation shake
table, of a scale model of one prototype church, before and after
retrofitting.

In the final phase of research, analysis of the data, development
of mathematical models, and vulnerability functions for various
parts of church buildings were used to create general recommendations
for retrofitting Byzantine churches.

The Church Of St. Nikita

The scale model of the Church of St. Nikita, constructed to test seismic stabilization techniques. Photo courtesy of the Institute of Earthquake Engineering and Engineering Seismology.

The prototype chosen for modeling and testing was the Church
of St. Nikita, located on Skopska Crna Gora, west of the village
of Banjani, near Skopje. While there is no record of its date of
construction, indirect evidence based on the church's frescoes (painted
by two prominent medieval fresco painters, Mihajilo and Evtikie)
suggests that St. Nikita was built in the early 14th century on
the foundations of an earlier, Byzantine Empire church.

The church, part of a monastic complex, is a single-dome structure
constructed in the shape of an inscribed cross. The walls are made
of tuff (a weak volcanic rock) alternating with layers of brick
set in lime mortar. Four interior brick columns support the massive
brick tambour and dome.

The test model of St. Nikita, built on the 5-by-5 meter shake table,
was 4.5 meters (14.8 feet) tall and weighed 21 tons. To create an
accurate model, the project team measured St. Nikita's response
to minor seismic activity in a series of simulation experiments,
then, using the results, constructed a model that duplicated the
real church's behavior during a seismic event.

To determine the actual seismic risk for St. Nikita, the team conducted
geophysical surveys and, based on historical records, estimated
the characteristics and return frequency of various types of earthquakes
for the region. (As a rule, larger quakes occur less frequently
than smaller ones.) These studies formed the basis for the computer-controlled
earthquake simulations used in testing the model.

The original, undamaged modelextensively instrumentedwas
subjected to three types of earthquakes of varying intensity and
duration. Twelve tests were performed, with intensity levels progressively
increased. During the testing, the dome vibrated strongly. The model's
first cracks appeared in the dome at a low intensity level, but
no cracks occurred in the lower part of the structure. The final
test at four times this level caused numerous cracks and the model's
structural failure.

Following these tests, the model was repaired by injection grouting
of cracks and by structural strengthening. Bolted horizontal steel
tie rods were incorporated at three levels within the rubble part
of the walls in the areas where wood timber belts originally existed
in the prototype.

At the base of the dome a horizontal band was attached. Vertical
steel ties were applied in the tambour and anchored to the main
walls. In addition, exterior walls were anchored to the foundation
with steel ties, and the spaces around the ties were filled with
grout to provide positive connections with the walls. Thus, in an
actual retrofit of a real church, no evidence of the engineering
changes would be visible.

The retrofitted model was subjected to the same earthquake simulations
used in the original tests. It was found that the maximum above-ground-level
forces decreased and the cracking pattern and crack location changed.
Displacement valuesthe amount of movement of building elementswere reduced by half.

To study the structure's behavior after cracking and to estimate
the damage from higher-than-expected earthquake intensities, the
earthquake amplitude was increased to a level that statistically
would occur in the region only once in a thousand years. This resulted
in some additional wall cracking and damage in the upper parts of
the church, but no structural damage.

According to Dr. Gavrilovic, the tests in Skopje "showed that in
the case of an earthquake of maximum expected intensity in the vicinity
of St. Nikita, structural failure of the church, as it now exists,
is likely and that seismic retrofitting should be undertaken."

Indeed, the experiments on the model of St. Nikita indicate that
the church's structural stability can be increased enough to prevent
structural damage during a major earthquake. The project also demonstrated
that through careful design, structural stabilization can be achieved
while the ethics of conservation are respected. The techniques used
would neither alter the external appearance of the church nor damage
its interior frescoes or other significant features.

It is expected that the analytical methods and retrofitting designs
developed during this project will be applicable to the many historic
structures of similar construction that are now at risk in earthquake
regions throughout the world.

William S. Ginell is Head of Architecture and Monument Conservation
Research in the GCI's Scientific Program.

GLOSSARY

Aisle One of several walkways of a basilica that extends
the length of the church in an east-west direction.

Apse Semicylindrical or polygonal extension on the east
end of a church.

Basilica Type of church having aisles, apse, nave, and
often a narthex.

Ductility The ability of a material or structure to deform
under stress without fracture.

Narthex Western entry area into a church leading to the
nave and aisles.

Nave High central aisle of a basilica.

Rubble Uncut stone and brick used as random fill between
two masonry walls.